The present invention relates to a method for detecting undersupply of a fuel gas and method for controlling a fuel cell in which the fuel gas is supplied to an anode on one surface of an electrolyte, and an oxygen-containing gas is supplied to a cathode on the other surface of the electrolyte for performing power generation.
For example, a solid polymer fuel cell employs an electrolyte electrode assembly (membrane electrode assembly) which includes anode and cathode, and an electrolyte (electrolyte membrane) interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane (proton ion exchange membrane). The electrolyte electrode assembly is sandwiched between separators. The electrolyte electrode assembly and the separators make up a unit of a fuel cell. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell stack, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
Normally, the amount of fuel gas supplied to the fuel cell is measured by a gas flow rate meter. The supply of the fuel gas to the fuel cell is constantly controlled such that the amount of the fuel gas corresponds to the load at the time of operating the fuel cell.
When the fuel cell is used in a vehicle application, in order to reduce the cost and size of the fuel cell, it is necessary to operate the fuel cell without any instruments for measuring the amount of the fuel gas supplied to the fuel cell.
However, since there is no means for detecting whether the sufficient amount of fuel gas for power generation is supplied to the fuel cell or not, the shortage of the stoichiometry (the shortage of the fuel gas) may occur undesirably, in particular, at the time of operating the fuel cell for the high load. Therefore, the power generation performance is significantly low.
In an attempt to solve the problem, Japanese laid-open patent publication No. 6-243882 discloses a method for stopping protection of a fuel cell power generation apparatus. According to the disclosure, a fuel cell stack is regarded as a plurality of cell assemblies each including a plurality of unit cells. The output voltages of the respective cell assemblies are detected. When the lowest detected voltage is decreased by a certain voltage, the protection of the fuel cell power generation apparatus is stopped regardless of the level of the electrical power required for the load.
However, in the above conventional technique, since the fuel cell stack is divided into cell assemblies, and the voltages of the respective cell assemblies are detected using a plurality of voltage detectors, and each of the detected values are compared with a predetermined value indicating a protection level. Therefore, the apparatus has a considerably complicated structure, and is large as a whole. Therefore, the cost of the apparatus is high.